[behnamoh]

Thanks for mentioning these points. Another explanation could be that _if_ these UFO's are in fact extraterrestrials with a technology advanced enough to travel to our planet, then it's plausible to assume that they also adapt their techniques based on our technological capabilities. In other words, if they are real and they know we have more advanced cameras compared to 100 years ago, chances are they are not going to reveal themselves to us as much as before, or they do that in situations where environment noise does not provide a high-res picture of them (like in the sea).